Optical components, for example, optical discs, are generally produced by a molding process, for example, injection molding. The injection molding process utilizes a movable mold that is closed under high pressure against a stationary mold to form a mold cavity therebetween having a shape approximately equal to the shape of the desired molded product. In molding optical components, for example, audio compact discs or memory compact discs, a thermoplastic disc material, for example, a polycarbonate resin, is heated to a molten phase and injected into the mold cavity. The molten material is injected into the center of the mold cavity, and the molten material flows radially to the peripheral boundary of the mold cavity. The molds are generally made of a metal material, for example, a stainless steel, and have a high thermal conductivity, thereby facilitating the rapid cooling of the molten disc material. After the molded disc in the cavity has cooled to a solid phase, the movable mold is withdrawn to open the mold and permit the molded disc to be removed therefrom.
As with any production process, the above molding process is continuously reexamined and optimized to shorten the molding cycle time, thereby providing a greater number of molded parts per operating shift. However, attempts to shorten the molding cycle time, especially in the process of making optical components, often results in an unacceptable degradation of the optical characteristics of the molded component.
One measure of the optical quality of a molded optical component is birefringence. Birefringence is the characteristic of having two indices of refraction with different values causing separation of a light beam passing through the material into two diverging beams. An unpredictable separation of a light beam in a molded optical component, such as an optical disc or lens, can cause errors and inaccuracies in the process of reading data from or writing data to the disc or through the lens. Further, it has been found that birefringence is directly proportional to orientation stresses existing within the molded component. Thus, in the injection molding process, birefringence is directly proportional to both flow induced orientation stress and stress caused by rapid cooling through the glass transition temperature with a large temperature gradient.
The literature suggests methods for reducing birefringence in molding discs normally used for optical components. For example, one method suggests molding components with predetermined magnitudes of stress present in the discs after molding, and those stresses are then subsequently removed by a post molding process, for example, an annealing process. Other methods utilize different temperature profiles across the mold during the injecting and cooling periods. Still other methods utilize insulating mechanisms within the mold to control the heat transfer and cooling of the injected resin.
U.S. Pat. No. 5,458,818 discloses an insulating insert that is placed within the mold to reduce residual stress and orientation in the molding process, thereby producing more uniform birefringence. The insert may be a single layer of insulating film or may have an outer skin layer bonded to the insulating layer. Preferably, the outer skin layer has a metallic layer, for example, electroless nickel and an intermediate bonding layer comprised of a porous matrix of metal particles to facilitate bonding of the hard, metallic skin layer with the insulating material. The metallic outer skin has a different coefficient of expansion than the intermediate porous layer or the insulating layer. Consequently, the heating and cooling cycles experienced with each of the tens of thousands of expected molding machine cycles will create a tendency for the metallic outer skin to separate from the insulating layer. In addition, the nickel outer skin is substantially softer than the stainless steel of the mold. Therefore, while there may be an improvement in birefringence, a mold with the above insulating insert has a disadvantage; namely, it is less durable and will have a shorter life than a stainless steel mold.
U.S. Pat. No. 5,324,473 discloses the use of one or more nonmetallic, thermal flow control, insulator elements that form the mold cavity surfaces. The insulator elements preferably have a thermal conductivity, density, and specific heat that is substantially less than the same properties of tool steel from which the mold components are made. Such materials are selected from the group consisting of quartz glass, "PYREX" glass, sapphire, and polyimide thermoplastic. Insulator elements comprise opposed, generally circular mold cavity side surfaces in the opposed mold halves; and ring shaped insulator elements located around the peripheral edge of the mold cavity, thereby defining a cylindrical wall of the mold cavity. The materials chosen for the edge elements have lower mathematical products of thermal conductivity, specific heat, and density than the materials chosen for the side surface insulator elements. The side surface elements have greater surface areas per volume of resin from which to transfer heat than do the narrow edge regions. The edge insulator elements provide reduced heat flow at the cylindrical corners of the cavity to slow heat transfer from the edges. Therefore, the edges have a heat transfer rate that is similar to that of the walls, thus preventing the corners of the molded article from solidifying much before the side surfaces and the cylindrical wall.
While the above non-metallic insulating inserts improve the thermal characteristics and consistency of the molded part, thereby substantially reducing birefringence, the nonmetallic inserts have two significant disadvantages. First, the nonmetallic inserts are softer than the steel molds and, do not have the strength and durability of the tool steel molds over tens of thousands of molding cycles. Therefore, the nonmetallic inserts have a shorter life than the steel molds and have the disadvantage of requiring more frequent replacement. Further, due to the general chemical similarity between the nonmetallic insulating inserts and the disc resin material being molded, the molded part may occasionally stick to the insulating insert after cooling, thereby making ejection of the molded part from the mold more difficult.
Due to differences in properties of insulative materials vis-a-vis the tool steel of traditional molds, trade-offs are encountered when insulators are used in the mold to reduce birefringence in the molded product. Insulators, while exhibiting lower thermal conductivity, specific heat and density than tool steel, which tend to reduce birefringence in the molded product, are generally softer than tool steel, causing greater wear, and hence require more frequent replacement, then molds made of only steel.
Consequently, there is a need for a mold system that improves on the disadvantages of the above described systems.